Lithium secondary batteries have received attention as high-capacity power sources for portable and other appliances. Further, lithium secondary batteries have recently been receiving attention as high-output power sources for electric vehicles and the like. Chemical batteries such as lithium secondary batteries usually have a separator that electrically insulates a positive electrode from a negative electrode and holds an electrolyte. In the case of a lithium secondary battery, a micro-porous film made of polyolefin (e.g., polyethylene, polypropylene, etc.) is mainly used as the separator. The electrode assembly of a cylindrical lithium secondary battery is produced by winding the positive electrode and the negative electrode, with the separator interposed between the two electrodes, into a cylindrical shape.
However, when a lithium secondary battery is stored in an environment at extremely high temperatures for an extended period of time, its separator made of a micro-porous film tends to shrink. If the separator shrinks, then the positive electrode and the negative electrode may physically come into contact with each other to cause an internal short-circuit. In view of the recent tendency of separators becoming thinner with an increase in lithium secondary battery capacity, preventing an internal short-circuit becomes particularly important. Once an internal short-circuit occurs, the short-circuit may expand due to Joule's heat generated by the short-circuit current, thereby resulting in overheating of the battery.
Thus, in the event of an internal short-circuit, in order to suppress such expansion of the short-circuit, Japanese Laid-Open Patent Publication No. Hei 7-220759 proposes forming a porous heat-resistant layer that contains an inorganic filler (solid fine particles) and a binder on an electrode active material layer. Alumina, silica, or the like is used as the inorganic filler. The inorganic filler is filled in the porous heat-resistant layer, and the filler particles are bonded to one another with a relatively small amount of a binder. Since the porous heat-resistant layer is resistant to shrinking even at high temperatures, it has the function of suppressing the overheating of the battery in the event of an internal short-circuit.
Recently, in the field of the power source for portable appliances, there is an increasing need for fast charge, and fast charge requires charging at a high rate (e.g., 1 hour-rate or less). In the case of a high-rate charge, the electrode plate expands and contracts significantly during charge/discharge and a large amount of gas is produced, compared with a low-rate charge (e.g., 1.5 hour-rate or more). Therefore, the electrode assembly is distorted, and the porous heat-resistant layer may break since the amount of the binder contained in the porous heat-resistant layer is relatively small and the bonding between filler particles is weak. In such cases, the function of the porous heat-resistant layer of suppressing the overheating of the battery in the event of an internal short-circuit is impaired.